Post-translational modifications by the Small Ubiquitin-like Modifier (SUMO) family of proteins have been established as critical events in the cellular response to a wide range of DNA damaging reagents and radiation. Both radiation and chemotherapy kill cancer cells by inducing DNA damage that leads to genomic instability and/or cell death. Therefore, DNA repair mechanisms, which are SUMOylation dependent, play a critical role in the resistance of cancer cells to these treatments. However, the mechanism of SUMOylation in DNA damage response is poorly understood. We hypothesize that SUMO isoform-specific receptor proteins mediate SUMO isoform-specific functions in the repair of DNA damage. The proposed studies are based on our Preliminary Studies, which show that inhibition of SUMO-dependent protein-protein interactions inhibited the repair of DNA double-strand breaks, and increased the sensitivity of tumor cells to radiation and chemotherapeutic drugs that induce DNA damage. The proposed research integrates several scientifically diverse but highly synergistic approaches, including structural and molecular biological methods, to investigate the role of SUMOylation in the repair of DNA double-strand breaks, and to elucidate the functions of different SUMO isoforms in these processes. These studies will also investigate the structural basis for SUMO isoform-specific recognition, which will provide information for designing inhibitors of SUMO isoform-specific functions. The proposed studies address an innovative concept, and could lead to the establishment of a new paradigm of targeting SUMO-dependent mechanisms for cancer therapy. PUBLIC HEALTH REVELANCE: The overall goal of this proposal is to investigate the role of SUMOylation in DNA damage response.